1. Field of the Invention
The present invention generally relates to door locks and more particularly to an improvement in an electromagnetic door lock device.
2. Prior Art
There are a large number of different types of electromagnetic door locks, most of which are complicated and expensive to manufacture, install and service and many of which do not perform efficiently. One particular lock is shown in U.S. Pat. No. 4,487,439 which issued Dec. 11, 1984, to William C. McFadden and is entitled Magnetic Shear Locking Methods and Apparatus. This device utilizes a locking mechanism which operates under shear. However, this device also utilizes a specially configured electromagnet. When an armature is attracted thereto during locking, a small central tab on the housing extends into a central armature dimple to prevent the armature and the door to which it is attached from moving. Such an electromagnet housing configuration is expensive to make. Moreover, great care must be made to align the armature and housing, specifically, the the central dimple and tab, otherwise the lock will malfunction. Of more importance, considerable shear stress can occur to the housing with resultant cracking and disruption of the electromagnet, particularly if an attempt is made to force the door open while it is in the locked position. Finally, the device is not adapted to a wide variety of applications.
An improvement over the above-described locking device is set forth in U.S. Pat. No. 4,826,223, the patentee of which is the present inventor. That patent discloses a door lock device which isolates the locking tab on a separate locking tab plate releasably connected to the side of the electromagnet. If the door is forced or jammed and the tab on the tab plate is bent or broken, no damage to the housing and electromagnet occurs and the tab plate can be easily replaced. Unfortunately, the tab may bind against the armature of the device when locked and the armature may not immediately release and unlock when the electromagnet is deenergized unless the relative positions of the armature and tab are first very carefully adjusted, which takes time and some skill.
There remains a need for an improvement in such an electromagnetic door lock device which will overcome the binding and armature release problem, will permit easy adjustment of the device and which will permit the tab plate to continue to be used, even with a tab thereof bent or broken, so that replacement parts need not be immediately furnished.